This invention relates, in general, to semiconductor processing and more particularly to methods of making semiconductor substrates that are very flat.
The semiconductor industry is designing integrated circuit (IC) devices that incorporate increasingly smaller and more complex geometries. As a result, the materials and equipment used to manufacture such devices are subject to increasingly tighter constraints. For example, the semiconductor substrates or wafers used to build the IC devices must have low concentrations of defects and must be extremely flat.
Techniques for making semiconductor substrates are well known. Once an ingot of semiconductor material has been grown and shaped, it is sawn into individual substrates followed by a lapping or grinding process to make them more flat and parallel. Next, the substrate edges are rounded using an edge grind process. After edge grind, the substrates are etched to remove any work damage and contamination. Finally, the substrates are polished on one or both sides to provide a starting substrate ready for IC device manufacture.
Typically, substrate flatness is characterized by parameters such as total thickness variation (TTV) and site focal plane deviation (SFPD). The TTV of a substrate is the difference between the minimum and maximum thickness values measured across the surface of the substrate. The SFPD is the greatest distance above or below a selected focal plane and is measured either using a front side reference or a back side reference. Typically, many sites on a substrate are measured for focal plane deviation and yield is determined by the number of sites meeting a specified focal plane deviation value (e.g., less than 0.5 micron). With a back side reference SFPD (which is a more stringent test than the front side reference SFPD), the focal plane deviation is calculated based on a reference plane that is parallel to the back side of the substrate and that contains the center of the site being measured.
Although progress has been made in achieving flatter semiconductor substrates, structures and methods are still needed that produce substrates with enhanced flatness in order to support the semiconductor industry's push towards IC designs with smaller and more complex geometries. Also, it would advantageous to produce such substrates in a cost effective and reproducible manner.